1. Field of the Invention
This invention relates to an idling speed control system for an outboard motor, particularly to an idling speed control system for an outboard motor for small boats
2. Description of the Related Art
Small motor-driven boats are generally equipped with a propulsion unit including an internal combustion engine, propeller shaft and propeller integrated into what is called an outboard motor or engine. The outboard motor is mounted on the outside of the boat and the output of the engine is transmitted to the propeller through a clutch and the propeller shaft. The boat can be propelled forward or backward by moving the clutch from Neutral to Forward or Reverse position.
The idling speed of this type of the engine is controlled by use of a secondary air supplier that supplies secondary air through a passage that is connected to the air intake pipe downstream of the throttle valve. The passage is equipped with a secondary air control valve and the desired idling speed is obtained by regulating the opening of the secondary air control valve.
The amount of secondary air required to achieve the desired idling speed varies with aged deterioration of the engine. It also differs with clutch position. This is because the idling speed differs between that when the clutch is in Neutral and that when it is in Forward or Reverse and the outboard engine is running forward or backward at very low speed, i.e., during trolling.
To give a specific example, say that the idling speed is 750 rpm when the clutch is in Neutral. When the clutch is then shifted into Forward or Reverse for low-speed trolling, since the hull acts to load and a quite low speed is required, the engine speed to fall to the trolling speed (herein defined as the idling speed during trolling) of around 650 rpm. The required amount of secondary air changes as a result.
With this, when the engine speed changes form the trolling speed to the idling speed and vice versa, as illustrated in FIG. 17, the engine speed may sometimes rise or drop sharply. The overshooting or undershooting of the engine speed from a desired speed due to the clutch change produce shock and hence, degrades feeling of the operator.
Further, if the operator of the outboard motor should replace the propeller, which is not uncommon, the resulting load change will change the engine speed and, accordingly, change the amount of secondary air required to achieve the desired idling speed.
As a result, when the manipulated value in the idling speed control is fixed to a value such that a desired idling speed is achieved, the stability of control is not satisfactory against the load change.
A possible technique to overcome the problem will be to determine the amount of secondary air required to achieve the desired idling speed through a learning control. However, the learning control is generally effective in steady state engine operation, but is less effective in the transient engine operation in which the clutch change results in switching of the idling speed to the trolling speed and vice versa. Thus, even if the learning control is introduced, this can not improve the control stability and can not surely suppress the overshooting or undershooting of engine speed due to the load change.
An object of the present invention is therefore to solve the problem by providing an idling speed control system for an outboard motor that is equipped with an internal combustion engine which supplies secondary air in such amount as to reduce difference between a desired idling speed determined in response to the clutch position and a detected engine speed, which can surely improve the control stability and suppress the overshooting or undershooting of engine speed due to the load change when the clutch is changed, thereby enabling to eliminate or reducing the shock to be felt by the operator during the clutch change and improving the feeling of the operator.
For realizing this object, there is provided a system for controlling an idling speed for an outboard motor mounted on a boat and equipped with an internal combustion engine whose output is connected to a propeller through a clutch such that the boat is propelled forward or reverse when the clutch is changed to a neutral position to a forward position or a reverse position, comprising: secondary air supplier that supplies secondary air trough a passage that is connected to an air intake pipe downstream of a throttle valve and that is equipped with a secondary air control valve such that amount of secondary air is supplied to the air intake pipe in response to an opening of the secondary air control valve; clutch position detecting means for detecting a position of the clutch; engine operating condition detecting means for detecting parameters indicative of operating conditions of the engine including at least an engine speed; desired value determining means for determining a desired idling speed based on the detected position of the clutch and for determining a desired secondary air supply amount such that a difference between the determined desired idling speed and the detected engine speed decreases; and valve controlling means for controlling the opening of the valve to a value that effects the desired secondary air supply amount; wherein the desired value determining means including: comparing means for calculating a change of the desired secondary air supply amount and for comparing the change with a predetermined value when it is determined based on the detected position of the clutch that the clutch is changed; change direction determining means for determining whether the change is in an increasing direction or in a decreasing direction; and correcting means for correcting the desired secondary air supply amount by a predetermined correction amount in the determined direction, when the change is greater than the predetermined value.